U.S. patent number 5,231,480 [Application Number 07/614,670] was granted by the patent office on 1993-07-27 for airborne imaging lidar system employing towed receiver or transmitter.
This patent grant is currently assigned to Kaman Aerospace Corporation. Invention is credited to Bobby L. Ulich.
United States Patent |
5,231,480 |
Ulich |
July 27, 1993 |
Airborne imaging lidar system employing towed receiver or
transmitter
Abstract
An imaging lidar apparatus for detecting and imaging an object
enveloped by a backscattering medium which is at least partially
transmitting to light is presented. The imaging lidar apparatus is
mounted on an airborne platform and including light pulse
generating means, reflected light pulse detection means and
computer control means. A discrete vehicle is towed by a cable
connected to the airborne platform. The discrete vehicle houses
optics for receiving or transmitting light pulses. Fiber optic
communication may be used to transmit the light pulses along the
cable between the airborne platform and the towed vehicle.
Inventors: |
Ulich; Bobby L. (Tucson,
AZ) |
Assignee: |
Kaman Aerospace Corporation
(Bloomfield, CT)
|
Family
ID: |
24462253 |
Appl.
No.: |
07/614,670 |
Filed: |
October 24, 1990 |
Current U.S.
Class: |
348/31;
356/5.04 |
Current CPC
Class: |
G01S
17/89 (20130101) |
Current International
Class: |
G01S
17/89 (20060101); G01S 17/00 (20060101); G01C
003/08 (); H04N 007/00 () |
Field of
Search: |
;358/95 ;356/4,5
;367/149,130 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Buczinski; Stephen C.
Attorney, Agent or Firm: Fishman, Dionne & Cantor
Claims
What is claimed is:
1. An imaging lidar apparatus for detecting and imaging an object
enveloped by a backscattering medium which is at least partially
transmitting to light, the imaging lidar apparatus being mounted on
an airborne platform and including light pulse generating means for
generating pulses of light toward a target, gated light pulse
detection means for detecting pulses of light reflected form the
target and image display means for displaying an image of the
detected target, and further including:
a discrete vehicle for housing projecting means for projecting
pulses of light generated by said light pulse generating means;
and
cable means for attaching the vehicle to the airborne platform and
towing the vehicle through the air or water, said cable means
including fiber optic means for optically transmitting the pulses
of light form said light pulse generating means to said projecting
means.
2. The apparatus of claim 1 wherein:
said pulses of light are projected by said projecting means toward
a target at an angle of less than 180.degree. with respect to said
airborne platform.
3. An imaging lidar apparatus for detecting and imaging an object
enveloped by a backscattering medium which is at least partially
transmitting to light, the imaging lidar apparatus being mounted on
an airborne platform and including light pulse generating means for
generating pulses of light toward a target, gated light pulse
detection means for detecting pulses of light reflected form the
target and image display means for displaying an image of the
detected target, and further including:
a discrete vehicle for housing receiving optics for initially
receiving reflected light pulses; and
cable means for attaching the vehicle to the platform and towing
the vehicle through the air or water, said cable means including
fiber optic means for optically transmitting the pulses of light
received form the receiving optics to said gated light pulse
detection means.
4. The apparatus of claim 3 wherein:
said pulses of light are projected by said projecting means toward
a target at an angle of less than 180.degree. with respect to said
airborne platform.
5. An imaging lidar apparatus for detecting and imaging an object
enveloped by a backscattering medium which is at least partially
transmitting to light, the imaging lidar apparatus being mounted on
an airborne platform and including light pulse generating means,
reflected light pulse detection means, image display means for
displaying an image of a target detected by the detection means and
computer control means and further including:
a discrete vehicle for housing said light pulse generating
means;
cable means for attaching the vehicle to the airborne platform and
towing the vehicle through the air or water; and
means for transmitting signals between said light pulse generating
means and said computer control means.
6. The apparatus of claim 5 wherein said light pulse generating
means includes projecting means and wherein:
pulses of light generated by said light pulse generating means are
projected by said projecting means toward a target at an angle of
less than 180.degree. with respect to said airborne platform.
7. An imaging lidar apparatus for detecting and imaging an object
enveloped by a backscattering medium which is at least partially
transmitting to light, the imaging lidar apparatus being mounted on
an airborne platform and including light pulse generating means,
reflected light pulse detection means and computer control means
and further including:
discrete vehicle for housing said light pulse detection means;
cable means for attaching the vehicle to the airborne platform and
towing the vehicle through the air or water; and
means for transmitting signals between said light pulse detection
means and said computer control means.
8. The apparatus of claim 7 wherein:
said pulses of light are projected by said projecting means at an
angle of less than 180.degree. toward an object.
Description
CROSS-REFERENCE TO RELATED APPLICATION
This application is related to an application entitled "Imaging
System Employing Bistatic Operation," Ser. No. 07/602,727 filed
Oct. 24, 1990 by R. Norris Keeler and Bobby L. Ulich, now
abandoned.
BACKGROUND OF THE INVENTION
This invention relates generally to a sensor system for remote
detection and imaging of objects in a backscattering medium such as
air or water. More particularly, this invention relates to a method
and apparatus for detecting, locating and/or imaging underwater
objects such as mines and submarines from an airborne platform
using a novel imaging lidar (light detection and ranging) system
which employs a method for improving the imaging of targets being
viewed in reflection using bistatic operation of the imaging lidar
system.
It is desirable in a number of military and civilian applications
to search a volume within a backscattering medium for the presence
of certain targets. For instance, moored or bottom mines deployed
in ocean shipping lanes are a hazard to navigating ships used both
for military and for commercial purposes. For other civilian
applications such as law enforcement on the ocean, it is desirable
to detect the presence of submerged fishing nets or drug-carrying
containers used in smuggling contraband. In or near harbors and
beaches, it is also desirable to detect submerged obstructions,
cables, pipelines, barrels, oil drums, etc. In strictly military
applications, anti-submarine warfare demands an effective means of
detecting and locating submarines.
Presently, cumbersome and time consuming wire line devices must be
used for detecting underwater targets from remote airborne
locations. These devices are lowered into the water and of course,
are easily subject to damage and loss. Also, wire line devices make
target searching relatively slow and can only detect targets
without providing visual imaging. An important and novel system for
remote detection and imaging of objects underwater (or objects
obscured by other backscattering media which are at least partially
transmitting to light such as ice, snow, fog dust and smoke) from
an airborne platform has been described in U.S. Pat. No. 4,862,257
and U.S. patent application Ser. No. 256,778 filed Oct. 12, 1988,
now U.S. Pat. No. 5,013,917, both of which are assigned to the
assignee hereof and incorporated herein by reference. The imaging
lidar system of U.S. Pat. No. 4,862,257 utilizes a laser to
generate short pulses of light with pulse widths on the order of
nanoseconds. The laser light is expanded by optics and projected
down toward the surface of the water and to an object or target.
U.S. application Ser. No. 256,778 relates to an imaging lidar
system intended for night vision.
Imaging lidar systems of the type described hereinabove are also
disclosed in commonly assigned U.S. patent application Ser. No.
420,247 filed Oct. 12, 1989 (now U.S. Pat. No. 4,964,721), and U.S.
patent application Ser. No. 364,860 filed Jun. 12, 1989 (now U.S.
Pat. No. 4,967,270, both of which are incorporated herein by
reference. USSN 420,247 relates to an imaging lidar system which
controls camera gating based on input from the aircraft onboard
altimeter and uses a computer to thereby adjust total time delay so
as to automatically track changing platform altitude. USSN 364,860
relates to a lidar system employing a plurality of gated cameras
which are individually triggered after preselected time delays to
obtain multiple subimages laterally across a target image. These
multiple subimages are then put together in a mosaic in a computer
to provide a complete image of a target plane preferably using only
a single light pulse.
USSN 565,631 filed Aug. 10, 1990 which is also assigned to the
assignee hereof and fully incorporated herein by reference, relates
to an airborne imaging lidar system which employs multiple pulsed
laser transmitters, multiple gated and intensified array camera
receivers, an optical scanner for increased field of regard, and a
computer for system control, automatic target detection and display
generation. USSN 565,631 provides a means for rapidly searching a
large volume of the backscattering medium (e.g., water) for
specified targets and improves upon prior art devices in
performance as a result of having more energy in each laser pulse
(due to simultaneous operation of multiple lasers) and a more
sensitive detection system using multiple cameras. The several
cameras may be utilized to image different range gates on a single
laser pulse or several cameras can be gated on at the same time to
provide independent pictures which can then be averaged to reduce
the noise level and improve sensitivity. Both of these improvements
result in higher signal-to-noise ratio and thus higher probability
of detection or greater range of depth capability.
In accordance with the imaging lidar systems of the type described
above, targets are detected by their contrast with the light
scattered or reflected back from the surroundings. If the target
falls within a gate, it will be seen as a bright spot if its
reflectivity is greater than the surrounding water; and either
indistinguishable or as a dark area if its reflectivity is equal to
or less than the surrounding water. If a target is above the gate,
obscuration occurs, and in this case, since the obscuration
represents a limiting case (no photons received) the signal to
noise ratio is determined by the intensity of the surrounding light
backscattered from the water.
Airborne imaging lidar systems fielded to date have been
monostatic. In other words, the system's transmitter (laser) and
receiver (camera) optics are colocated and coaxial. In a monostatic
lidar system, the light scattered back from the gated area returns
along the same path as it started from the transmitter. This
180.degree. backscatter occurs at a peak in amplitude, and thus
represents a maximum which occurs in backscattered light. This is
the optimum arrangement for objects viewed in obscuration. As
described in "Marine Optics", N. G. Jerlov, Elsevier Oceanography
Series 14, p. 34, Elsevier, New York (1976), this peak in
backscattering is symmetric around 180.degree.. The intensity of
this backscattering can decrease an order of magnitude at
deflections as small as +/- 10.degree. from 180.degree.. As a
result, the monostatic imaging lidar systems of the prior art are
not well suited for imaging a target when viewed in reflection.
SUMMARY OF THE INVENTION
The above-discussed and other problems and deficiencies of the
prior art are overcome or alleviated by the imaging lidar system of
the present invention which is adapted to decrease the
backscattering at the receiver when a target is viewed in
reflection and to increase the backscattered reflection when the
target is viewed in obscuration by operating the airborne lidar
imaging system bistatically in the former case, and monostatically
in the latter case.
In accordance with a first embodiment of the present invention, a
retractible prism and remote reflecting mirror are used to direct
the laser transmitter beam downward. The reflecting mirror is
offset so that there is a finite angle between the transmitter
optical path and the path of the light reflected back into the CCD
framing camera. The angle can be varied by moving the reflecting
mirror along a track or rail with the appropriate adjustment to the
mirror so that the transmitter beam is completely captured and
directed downward to illuminate the area viewed by the camera.
In a second embodiment of the present invention, the camera is
placed on runners and displaced from the transmitter beam. A
control is inserted so that the transmitter optics are directed to
the area imaged by the camera.
In addition, two variations of these two embodiments are provided
in which first, the camera optics move in response to the movement
of the transmitter beam, and second, the camera optic are directed
to view the area illuminated by the laser transmitter as the camera
moves away from the location of the transmitter. In all of these
preferred embodiments, the platform for the transmitter (and/or
receiver) is an airborne system.
Still other embodiments of this invention are presented wherein a
discrete vehicle towed by a cable is used to achieve bistatic
imaging.
In several of the above-described embodiments, fiber optic
communication may be utilized to achieve the required bistatic
viewing angles.
The present invention is particularly useful in imaging targets
which can be observed in reflection or in obscuration and for which
it would be desirable to maximize the ambient backscatter (target
in obscuration), or on the other hand, minimize the ambient
backscatter (target in reflection). Of course, multiple cameras can
be employed so that simultaneous monostatic and bistatic operation
can be achieved, providing optimized detection in both reflection
and obscuration modes.
The above-discussed and other features and advantages of the
present invention will be appreciated and understood by those of
ordinary skill in the art from the following detailed description
and drawings:
BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, wherein like elements are numbered
alike in the several FIGURES:
FIG. 1 is a graph depicting the amplitude of volume backscatter as
a function of scattering angle;
FIGS. 2A and 2B are graphical representations showing backscattered
light for a single pulse in respective monostatic and bistatic
lidar systems;
FIGS. 3A through 3D are graphical representations showing
backscattered light for a target in reflection for monostatic and
bistatic lidar systems, respectively;
FIGS. 4A and 4B are schematic diagrams of monostatic and bistatic
imaging lidar systems, respectively;
FIGS. 5A and 5B are graphical representations showing backscattered
light for a single pulse in obscuration for monostatic and bistatic
lidar systems, respectively;
FIGS. 6A through 6D are graphical representations of the target
signatures of FIGS. 5A and 5B, respectively, shown on a video
frame;
FIGS. 7A and 7B are schematic diagrams depicting the light paths
for the respective targets of FIGS. 5A and 5B;
FIG. 8 is a schematic diagram of an imaging lidar system capable of
both monostatic and bistatic operation;
FIG. 9 is a diagrammatic view of an alternative embodiment of the
imaging lidar system of the present invention installed on a
helicopter;
FIG. 10 is a diagrammatic view of still another embodiment of this
invention which employs fiber optic communication; and
FIG. 11 is a diagrammatic view of yet another embodiment of this
invention which employs a discrete vehicle towed by a cable.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring first to FIG. 1, and in accordance with the present
invention, it is shown that to decrease the backscatter by a factor
of ten (one order of magnitude) for observation of a target in
reflection, the transmitter (e.g., lasers) and receiver (e.g.
camera) must be in the bistatic mode with path separations in the
water of 10.degree.. For a decrease of a factor of twenty, a path
separation of 40.degree. is required. Although theoretically a
decrease of close to two orders of magnitude could be obtained with
a 90.degree. angle, this would present practical difficulties. In
addition to the maximum shown in FIG. 1, additional effects can
also be represent over angles close to 180.degree., but the
presence of such effects does not alter the qualitative performance
of the present invention.
Referring to FIGS. 2A-B, 3A-B and 4A-B, the effects of separating
the transmitter and receiver are shown wherein two lidar systems
are seen operating side by side in a monostatic mode (FIGS. 2A, 3A
and 4A) and a bistatic mode (FIGS. 2B, 3B and 4B). In FIGS. 2A, 3A
and 4A, the lidar system with pulsed laser transmitter 10 and gated
intensified charge coupled device (ICCD) camera 12 (FIG. 4A) is
illuminating and viewing a target 14 underneath the ocean surface.
The illuminating light 16 and returned light 18 are coaxial; that
is, the scattering angle of the returned light is 180.degree. (with
respect to the Zenith direction). FIG. 2A is a graph 20 taken from
FIG. 1, showing that the light backscattered from the ocean
surrounding the target is at a maximum, and this is reflected in
trace 22 across one of the video frames 24 (see FIG. 3A) from the
illumination of target 14 by a single pulse. The trace 22 taken
across this video frame 24 shows a level of "noise" 26 representing
return from the sea water, and a "signal" 28 representing return
from the target.
Referring now to FIGS. 2B, 3B and 4B, the bistatic system consists
of a separately located laser transmitter 30 and camera 32. A
target 34 is shown at the same depth and is physically identical to
target 14. The effect of the separation of camera 32 and laser
transmitter 30 is shown in the graph 36 (FIG. 2B). Light 38
scattered back from the illuminated area toward the transmitting
laser is at the same intensity as the light received at the camera
of the monostatic system, and this is shown as point 40 on graph
36. However, the light received at the bistatic camera returning
along path 42 at an angle .theta. from the path 44 of the light
transmitted downward is of lower intensity, as represented by the
point 46 on graph 36. A trace 48 across the video frame 50
corresponding to this situation (FIG. 3B) shows a decreased "noise"
level 52 and a "signal" 54 which is comparable to the signal 28
which was coaxial with the illuminating light beam in the
monostatic case (see FIG. 3A). The reason that signals 54 and 28
are roughly equal in magnitude is that the target is a diffuse
lambertian reflector. If the target had been a specular reflector
of high reflectivity (e.g., a mirror) directed back at the
transmitters 10 and 30, the signal 28 would have been relatively
intense, but the signal 54 would have been negligible. Note also
that in accordance with this invention, the bistatic system (FIG.
4B) can be configured so that the laser transmitter 30 illuminates
the same volume viewed by the camera 32 (as best shown in FIG.
9).
Referring to FIGS. 5A-B, 6A-B and 7A-B, a comparison between the
monostatic (FIGS. 5A, 6A and 7A) and bistatic (FIGS. 5B, 6B and 7B)
imaging of targets in obscuration is shown. In the case of
monostatic operation, the transmitted light 60 and backscattered
light 62 are coaxial; that is, the light 62 returns to the camera
along the same path that the pulsed illuminating light 60 arrived.
Thus, the scattering angle is 180.degree.. This is the point for
maximum scattering return from the ocean as shown by the graph 64
in FIG. 5A. The ICCD camera gate is timed to observe the volume 66
which is below the object 68 to be observed. Thus, the object 68 is
seen in obscuration, since all light which would have returned from
the imaged area 66 to the camera, and passing through the target
will be blocked or obscured. This appears on the video screen 70 as
a dark spot 72 which is highlighted by the surrounding backscatter
originating from that part of 66 not obscured by the target above
it. The trace 74 is plotted showing the surrounding "noise" 76 and
signal 78. In this instance, contrast is provided by the value of
the surrounding backscattered light, so that in this case the noise
becomes the signal and the signal is merely the absence of noise.
For the bistatic case, (FIGS. 5B, 6B and 7B), the backscatter
available to provide contrast with the absence of signal from the
obscured region is reduced thereby decreasing the effective
"signal" to "noise" ratio. In this bistatic case, the pulsed laser
illumination 80 illuminates the area imaged by the ICCD camera, The
light returns at 82 to the camera which is now no longer coaxial
with the pulsed laser transmitter. The angular separation of the
light rays is .theta.. The light which would have returned to the
camera through the space now occupied by the target 87 is obscured.
An obscuration volume 86 results similar to 66 in the monostatic
case. As can be seen from referring to graph 86, the intensity of
the backscatter is reduced, providing lesser contrast with the
shadow area caused by the obscuration 88. This can be seen on the
video frame 90. A scan 97 across this frame shows the ambient
backscatter 94 decreased, while the signal level 96 which is
associated with the target obscuration remains relatively constant,
roughly equivalent to 78.
Referring to FIG. 8, a first preferred embodiment for achieving
bistatic configuration for an imaging lidar system is shown
generally at 98 wherein the transmitter and receiver are mounted
for monostatic (coaxial) sensing; with the addition of a movable
mirror to alter the trajectory of the pulsed light rays from the
transmitter. The pulsed laser transmitter 100 and ICCD camera 102
are mounted on an aircraft airframe 104. A rail 106 is provided for
a housing 108 of a turning prism and output beam steering optics
110 (e.g. mirror) so that the location of the effective origin of
the output pulse can be varied. A control coupling 112 for the
output optics is provided with input from the aircraft altimeter,
so that the pulsed laser transmitter can continue to illuminate the
volume of the ocean viewed by the ICCD camera, as altitude and
distance between transmitter and receiver are varied (see U.S.
patent application Ser. No. 420,247 which has been incorporated
herein by reference). The transmitted light pulses are initially
directed away from a 180.degree. degree trajectory by a first prism
113 and then directed to beam steering optics 110 by a second prism
114. The redirected output beam 115 is transmitted downwardly and
returns to the camera 102 as the backscattered pulse 116. Of
course, housing 108 is movable and steering optic 110 is pivotable
to alter the trajectory of the transmitted pulsed light as desired.
Moreover, the mirror 113 may be removed or disengaged to permit
conventional coaxial (monostatic) operation.
Turning now to FIG. 9, alternative arrangements for both bistatic
and monostatic imaging lidars are shown mounted on a helicopter
118. In a first of these embodiments, a pair of spaced receivers
120 and 122 are used in conjunction with a transmitter 124 wherein
receiver 120 is used in a bistatic mode and receiver 122 is used in
a monostatic mode. As discussed in detail above, the bistatic
arrangement may be useful for detecting targets in reflection. In
this configuration the camera 120 is physically separated in the
horizontal plane from the laser beam projection optics 124 and
their relative lines of sight are tilted to intersect at the target
search depth. In this case, the volume backscatter angle is not
180.degree. (with respect to the zenith direction) but is less than
180.degree.. The result will be appreciably reduced backscatter
light levels from the water 126 volume but no reduction in the
target 128 reflection intensity. Thus, the SNR will be improved by
avoiding the strong peaking of light backscatter at 180.degree.. As
also mentioned, for shadow detection, the coaxial mode
(180.degree.), and hence camera 122, are preferred since this gives
the highest SNR for that mode. Thus, the system of FIG. 9 employs
two sets of cameras 120, 122, one camera (e.g., 122) near the laser
for optimized shadow detection and one camera (e.g., 120) separated
for optimum reflection detection. Also, and for increased
flexibility, camera 120 may be mounted on rails or rollers 125 so
that it will be movable in the horizontal direction and the
distance between transmitter 124 and camera 120 may be easily
varied. In still another alternative embodiment, camera 122 may be
removed and only movable camera 120 used. Camera 120 would then be
movable between a first position coaxial with transmitter 124
(monostatic) and a plurality of second positions horizontally
displaced from transmitter 124 (bistatic).
In addition, the transmitter 124 and receivers 120, 122 are
compensated in the three aircraft axes for roll, pitch and yaw
during flight, thereby maintaining a constant, boresighted angular
relationship between the transmitter, receiver and the target area
of interest. This method will compensate for changes in aircraft
flight conditions as well as for external factors such as wind
gusts. A preferred compensating system is disclosed in FIG. 3 of
USSN 565,631 (which has been incorporated herein by reference).
In still another alternative embodiment shown in FIG. 10, the
electronics 130 for the gated camera are mounted near the laser
transmitter 124 in the lidar system housing 132. When bistatic
viewing is desired, a bundle of optical fibers 134 terminating at
horizontally displaced receiving optics 136 is used to transmit
received pulses of light to the lidar system camera sensor for
detection. Of course, fiber optics may also be used conversely. In
this latter case, element 136 comprises a projector; element 130
comprises the laser transmitter and element 124 comprises the
receiver. Thus, the transmitter 130 will transmit pulses of light
along at least one optical fiber 134 for projection downwardly
through projection optics 136. Reflected pulses will then be
received by receiver 124.
Referring now to FIG. 11, still two additional embodiments of the
present invention are schematically shown. In a first of these
additional embodiments, the airborne platform 150 (i.e.,
helicopter) includes a lidar system 151 having gated camera
receiver 152 conventionally positioned for 180 degree viewing. In
accordance with the present invention, the pulsed laser projector
154 is displaced horizontally from receiver 152 which is housed in
a discrete vehicle 156 pulled along by platform 150 using a
suitable cable 158. Vehicle 156 may be towed by platform 150
through the air or under water 170. Preferably, the pulsed laser
transmitter 160 is actually housed in lidar system 151 with an
optical fiber running along cable 158 so as to optically
interconnect transmitter 160 to projector 154. Fins 162 are
provided on vehicle 156 for stabilization purposes. As is clear
from a review of FIG. 11, the towed transmitter vehicle 166 will
provide a field of view 164 which is angled (when compared to the
field of view 166 provided by receiver 152) so as to result in the
lidar system 151 viewing the target 168 (under water 170) at a
bistatic angle.
In a second embodiment of these alternative embodiments, element
152 comprises a pulsed laser transmitter and element 160 comprises
the gated camera electronics with a bundle of optical fibers
running along cable 158 to receiving optics 154 on towed vehicle
156.
In still other variations of the embodiments of FIG. 11, the actual
pulsed laser transmitter or gated camera receiver may be housed in
the towed vehicle 156 (thus, element 154 would function either as a
transmitter or receiver). Communication between computer control
means in the lidar system 151 on airborne platform 150 and either
of the transmitter 154 or receiver 154 on vehicle 156 may be
accomplished by any known hardwired technique (e.g., along towing
cable 158) or wireless technique (e.g., radio waves).
It will be appreciated that the imaging lidar systems depicted in
FIG. 11 are novel not only in providing a means of accomplishing
bistatic operation; but also in their overall configuration wherein
either the transmitter means or receiver means are housed in a
discrete vehicle towed by the airborne platform and wherein optical
fibers may be used to optically interconnect the lidar system 151
to devices housed in vehicle 156.
It will be appreciated that a lidar imaging system that has been
described above has been described with improved ability to image
undersea targets, and provide a better signal to noise ratio and
probability of detection. Of course, any desired imaging lidar
system may be employed including systems incorporating multiple
lasers, multiple cameras, etc.
While preferred embodiments have been shown and described, various
modifications and substitutions may be made thereto without
departing from the spirit and scope of the invention. Accordingly,
it is to be understood that the present invention has been
described by way of illustrations and not limitation.
* * * * *